Ferritic stainless steel and method of manufacturing the same

ABSTRACT

The present disclosure relates to a ferritic stainless steel and fabrication method of a ferritic stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % of more to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe and other inevitable impurities.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims benefit of priority of Korean Patent Application No. 10-2011-0027104 filed on Mar. 25, 2011, and Korean Patent Application No. 10-2011-0027105 filed on Mar. 25, 2011. The entire contents of both applications are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a ferritic stainless steel which can be used for the exhaust manifold of vehicles and a method of manufacturing a ferritic stainless steel which can be used for the exhaust manifold of vehicles.

2. Description of the Related Art

Recently, the laws about discharging toxic substances in exhaust gases have been in force in many countries, against seriousness of environmental problems due to the exhaust gases from vehicles. A technology for improving performance of purifying the exhaust gas, using a catalyst, in consideration of the trend, has been the focus. The higher the temperature, the more the purifying reaction of NOx, HC, and CO increases. Therefore, in order to reduce discharging of pollutants, it has been a trend to continuously increase the temperature of the exhaust gas, and accordingly, it is strongly required to improve high-temperature characteristics of the parts constituting an exhaust system controlling the exhaust gas.

An exhaust manifold is a part that collects an exhaust gas from the cylinders in an engine and discharges the exhaust gas to exhaust pipes. Since the temperature of the exhaust gas reaches up to 900° C., the exhaust manifold is a part requiring oxidation resistance, high-temperature strength, and thermal fatigue property. In the related art, although nodular cast iron has been used as a material for the exhaust manifold, it has been replaced with ferritic stainless steel by a request for increase in temperature of an exhaust gas and decrease in weight of parts. Further, recently, as a turbo is mounted and an engine is downsized to improve fuel efficiency of vehicles, the temperature of an exhaust gas is expected to increase 30° C. to 50° C., as compared with the existing vehicles.

Therefore, 429EM, 441, and 444, the type of ferritic stainless steel used for the exhaust manifold in the related art cannot satisfy the product quality of customers, such that various studies about ferritic stainless steel having improved high-temperature performance have been conducted.

The present disclosure is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.

SUMMARY

In one aspect, the present disclosure provides a ferritic stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe and other inevitable impurities.

In another aspect, the present disclosure provides a fabrication method of a ferritic stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe and other inevitable impurities, wherein the fabrication method of the ferritic stainless steel comprises: providing a slab; heating the slab; hot annealing; cold annealing; and cold rolling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a test result of an influence of Mo and W on high-temperature strength.

FIG. 2A is an optical microscopic picture of a hot-annealed structure of Mo-added steel.

FIG. 2B is an optical microscopic picture of a hot-annealed structure of Mo+W-added steel.

FIG. 3 is a graph testing a thermal fatigue property of ferritic stainless steel according to the addition amount of Mo and W.

FIG. 4 is a graph testing high-temperature oxidation resistance of ferritic stainless steel according to the addition amount of Mo and W.

FIG. 5 is a graph testing high-temperature salt corrosion resistance of ferritic stainless steel according to the addition amount of Mo and W.

FIG. 6 is a flowchart schematically illustrating a fabrication method of a ferritic stainless steel according to one embodiment of the present invention.

FIG. 7 is a graph showing the grain size of ferritic stainless steel according to slab heating temperature.

FIG. 8 is a graph showing average r-bar values of ferritic stainless steel according to slab heating temperature.

FIG. 9 is a graph showing average r-bar values according to hot-annealing temperature in hot annealing.

FIG. 10 is a graph showing high-temperature tensile strength according to cold-annealing temperature in cold annealing.

FIG. 11 is a graph showing average r-bar values according to cold-annealing temperature/hot-annealing temperature.

FIG. 12 is a graph showing high-temperature tensile strength according to cold-annealing temperature/hot-annealing temperature.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Features of the present disclosure and methods to achieve them will be clear from exemplary embodiments described below in detail and with reference to the accompanying drawings. However, the present disclosure is not limited to the embodiments described hereafter and may be implemented in various ways.

Provided is a ferritic stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe and other inevitable impurities.

In some embodiments, Mo is 0.8 wt % or less, by weight %.

In some embodiments, a hot-annealed structure of the ferritic stainless steel comprises a sigma phase of 5% or less.

In some embodiments, W is 3 wt % or more to 6 wt % or less, by weight %.

In some embodiments, Mo wt %+0.83W wt % is 3.5 wt % or more to 5 wt % or less, by weight %.

In some embodiments, [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)] is 19.5 or more to 32 or less.

In some embodiments, ductile-brittleness transition temperature (DBTT) is 90° C. or less.

In some embodiments, the ferritic stainless steel satisfies the following equation: −184.6+3.2 (Cr wt %)+27.5(Mo wt %)+4243.4(C wt %+N wt %)−295.6(Al wt %)+0.9[Nb wt %/(C wt %+N wt %)] 90.

Also provided is a fabrication method of a ferritic stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe and other inevitable impurities,

wherein, the fabrication method of the ferritic stainless steel comprises: providing a slab; heating the slab; hot annealing; cold annealing; and cold rolling.

In some embodiments, Mo is 0.8 wt % or less, by weight %.

In some embodiments, W is 3 wt % or more to 6 wt % of less, by weight %.

In some embodiments, Mo wt %+0.83W wt % is 3.5 wt % or more to 5 wt % or less, by weight %.

In some embodiments, [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)] is 19.5 or more to 32 or less.

In some embodiments, ductile-brittleness transition temperature (DBTT(° C.)) is 90° C. or less.

In some embodiments, the ferritic stainless steel satisfies the following equation: −184.6+3.2(Cr wt %)+27.5(Mo wt %)+4243.4(C wt %+N wt %)−295.6(Al wt %)+0.9[Nb wt %/(C wt %+N wt %)] 90.

In some embodiments, grain size is ASTM No. 3 or more. In some embodiments, heating the slab has slab heating temperature that is 1180° C. or more to 1240° C. or less, with respect to slab temperature.

In some embodiments, hot annealing has a hot-annealing temperature that is 1020° C. or more to 1070° C. or less, with respect to strip temperature.

In some embodiments, cold annealing has a cold-annealing temperature that is 1030° C. or more to 1080° C. or less, with respect to strip temperature.

In some embodiments, (cold-annealing temperature)/(hot-annealing temperature) is 1.0 or more to 1.1 or less, wherein the cold-annealing temperature is the temperature in cold-annealing and the hot-annealing temperature is the temperature in hot annealing.

In some embodiments, cold rolling has a cold rolling temperature that is room temperature.

High-temperature oxidation resistance, high-temperature salt corrosion resistance, high-temperature strength, thermal fatigue property, and formability of ferritic stainless steel may be influenced by the elements contained in the ferritic stainless steel and the addition amount of the element. Hereafter, the component systems constituting the ferritic stainless steel of the present disclosure are described in more detail. The following component systems are expressed by weight %.

C may be above 0 wt % to 0.01 wt % or less in the ferritic stainless steel. Since C can increase room-temperature strength of the ferritic stainless steel, C can be added. On the contrary, when the amount of C is above 0.01 wt %, the room-temperature strength of the ferritic stainless steel may be increased, while high-temperature strength and ductility, machinability, and toughness at room temperature may be relatively decreased. Therefore, C may be above 0 wt % to 0.01 wt % or less, such as 0.005 wt % or less.

Si may be above 0 wt % to 0.5 wt % or less in the ferritic stainless steel. Si functions as a deoxidizer for a molten metal state of the ferritic stainless steel. Further, Si may improve oxidation resistance of the ferritic stainless steel. On the contrary, when Si is above 0.5 wt %, hardness of the ferritic stainless steel may be increased by solid solution hardening of Si, such that elongation and machinability of the ferritic stainless steel may be decreased. Therefore, Si may be above 0 wt % to 0.5 wt % or less.

Mn may be above 0 wt % to 2.0 wt % or less in the ferritic stainless steel. A scale may be produced at high temperature when the ferritic stainless steel is used as a material of the exhaust manifold of vehicles. In this case, the produced scale may be easily separated and the separated scale may flow into a catalytic converter and block the passage of the catalytic converter. Therefore, the ferritic stainless steel may have separation resistance for the scale, such that it may comprise Mn for the separation resistance. On the other hand, when Mn is above 2.0 wt %, MnS may be produced by reaction of Mn and S. MnS may have an adverse influence on the corrosion resistance of the ferritic stainless steel. Therefore, Mn may be above 0 wt % to 2.0 wt % or less.

Although P can increase the strength of the ferritic stainless steel, it may decrease the machinability. Further, P may be considered an impurity in the steelmaking process of the ferritic stainless steel, so P may be reduced. On the other hand, excessively decreasing P in processes is inefficient in terms of refining cost or productivity. Therefore, P may be 0 wt % or more to 0.04 wt % or less.

S may be 0 wt % or more to 0.02 wt % or less in the ferritic stainless steel. S may exist as an inclusion in the ferritic stainless steel or may function as an impurity that decreases the corrosion resistance. Therefore, although the amount of S may be reduced to improve corrosion resistance of the ferritic stainless steel, it may be inefficient in terms of cost and time to excessively reduce S in the process. Accordingly, S may be 0 wt % or more to 0.02 wt % or less, such as 0.003 wt % or less.

Cr may be 12 wt % or more to 19 wt % or less in the ferritic stainless steel. Cr is an alloy element that may improve corrosion resistance and oxidation resistance of the ferritic stainless steel. The ferritic stainless steel may not have sufficient corrosion resistance when the amount of Cr is low, therefore Cr may be 12 wt % or more. On the other hand, when the amount of Cr is above 19 wt %, the corrosion resistance of the ferritic stainless steel may be improved, whereas the strength may be excessively increased, such that the elongation and a shock property may be decreased. Therefore, Cr may be 12 wt % or more to 19 wt % or less.

Ti may be 0 wt % or more to 0.3 wt % or less in the ferritic stainless steel. Ti is an alloy element that may be added to improve the high-temperature strength and intergranular corrosion resistance of the ferritic stainless steel. When the amount of Ti in the ferritic stainless steel is above 0.3 wt %, steelmaking inclusion may increase and a surface defect, such as scab, may be generated and a nozzle may be clogged in continuous casting, such that the process efficiency may be decreased. Further, with the increase of solid solution Ti, the elongation and low-temperature shock property of the ferritic stainless steel may be decreased. Further, when Nb is added with Ti in the ferritic stainless steel and the ferritic stainless steel is used at high temperature for a long period of time, Fe3Nb3C carbide may be educed and coarsening may occur, such that high-temperature deterioration may be caused. Therefore, Ti may be 0 wt % or more to 0.3 wt % or less.

N may be above 0 wt % to 0.01 wt % or less in the ferritic stainless steel. Although N, similar to C, can increase the strength of the ferritic stainless steel, it may decrease ductility and machinability. Therefore, N may be above 0 wt % to 0.01 wt % or less to ensure sufficient elongation and machinability of welded portions, such as 0.007 wt % or less of N.

Mo may be 0 wt % or more to 1.0 wt % or less in the ferritic stainless steel, such as 0.8 wt % or less of Mo. When Mo is 0.8 wt % or less, the hot-annealed structure of the ferritic stainless steel may comprise a sigma phase of 5% or less.

W may be 2 wt % or more to 7 wt % or less in the ferritic stainless steel, such as 3 wt % or more to 6 wt % or less.

Various studies and efforts, such as addition of Mo, have been made in order to affect the high-temperature strength of ferritic stainless steel. In a method of addition of Mo, when Mo in ferritic stainless steel is 3 wt % or more, a sigma phase of ferritic stainless steel is produced. The sigma phase may not only cause a defect in manufacturing ferritic stainless steel, but may cause a decline in durability when ferritic stainless steel is used for the exhaust manifold of vehicles. It is possible to prevent a sigma phase from being produced by reducing Mo in the ferritic stainless steel according to the present disclosure. Further, the amount of Mo may be 1 wt % or less in the ferritic stainless steel according to the present disclosure to ensure the high-temperature strength. Mo may be 0.8 wt % or less.

In the steelmaking process of the ferritic stainless steel, since the steelmaking process is performed in large quantities, it is not easy to control the amount of substances within very small level, such that an effort for controlling the amount may be inefficient. On the other hand, since the elements, such as Mo, are expensive raw materials controlling the addition amount of Mo may reduce the manufacturing cost. Therefore, it may be possible to keep the properties of the ferritic stainless steel and improve the process efficiency by controlling the amount of Mo at 0.8 wt % or less.

When the amount of W is less than 2 wt %, the produced amount of nano-sized fine extracts, such as Fe2W, and the solid solution amount of W in a matrix may be decreased, such that it may be difficult to provide the ferritic stainless steel with sufficient high-temperature strength and thermal fatigue property. Further, when the amount of W is above 7 wt %, the cost of raw materials of the ferritic stainless steel may increase and a large amount of Fe2W may be produced in the ferritic stainless steel, which may be disadvantageous in line threading and may reduce production efficiency, and may decrease weldability and formability. The ferritic stainless steel shows tensile strength of 40 MPa or more in a high-temperature tensile strength test at 900° C. by further comprising W, such that it can be used for the exhaust manifold of vehicles which require high strength at high temperature.

When the amount of Mo is 1.0 wt % or less, such as 0.8 wt % or less, the ferritic stainless steel may further comprise W in order to ensure high-temperature oxidation resistance, high-temperature salt corrosion resistance, high-temperature strength, and a thermal fatigue property. Considering the influence on the high-temperature strength of the ferritic stainless steel, the relationship between the two elements, Mo and W, may be expressed as Mo wt %+0.83W wt %=3.5 wt % or more to 5 wt % or less. When Mo wt %+0.83W wt % is less than 3.5 wt %, the properties, high-temperature strength, high-temperature fatigue lifespan, high-temperature oxidation resistance, and high-temperature salt corrosion resistance, of the ferritic stainless steel may be decreased, and when it is above 5 wt %, the high-temperature properties may be excellent, but the elongation, which is a factor of room-temperature machinability, may be decreased, and toughness of the welded portions and the mother material may also be decreased.

The ferritic stainless steel may comprise Mo wt %+0.83W wt % of 3.5 wt % or more to 5 wt % or less. For Mo and W in the ferritic stainless steel, when the value of Mo wt %+0.83W wt % is less than 3.5 wt %, it may be difficult to provide the ferritic stainless steel with sufficient high-temperature strength and thermal fatigue property in order to be used for the exhaust manifold of vehicles. When Mo wt %+0.83W wt % of less than 3.5 wt % is in the ferritic stainless steel, the maximum available temperature of an exhaust manifold of a vehicle which is made of the ferritic stainless steel may be 900° C. or less, such that it is not available at higher temperature. Further, when the value of Mo wt %+0.83W wt % is above 5 wt %, a problem occurs in the line threading of the ferritic stainless steel, such that productivity may be decreased, and formability and weldability may also be decreased.

Ti may be 0 wt % or more to 0.3 wt % or less, Nb may be above 0 wt % to 0.6 wt % or less, N may be above 0 wt % to 0.01 wt % or less, and Al may be 0 wt % or more to 0.1 wt % or less, in the ferritic stainless steel. The relationship between the elements, [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)], may be 19.5 or more to 32 or less.

The ferritic stainless steel may comprise a predetermined amount of Ti and Nb. When the amount of Ti and Nb is less than a predetermined level, granular corrosion may occur at welding heat-influenced portions, or the high-temperature strength and thermal fatigue property may be decreased. Therefore, the amount of Ti and Nb can be controlled such that [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)] may be 19.5 or more. On the other hand, when [(Ti+1/2Nb)/(C+N)] is above 32, it may be advantageous in high-temperature properties of the ferritic stainless steel, the amount of solid solution Nb is excessively increased, such that room-temperature elongation, toughness, and machinability may be decreased. Therefore, [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)] may be 19.5 or more to 32 or less.

Hereafter, the present disclosure is described with reference to embodiments and comparative embodiments, which is for illustrating the present disclosure. The present disclosure is not limited to the following embodiments and comparative embodiments.

1. Manufacturing of Sample

Table 1 shows chemical components of samples used in the embodiments and comparative embodiments. Referring to Table 1, the embodiments and comparative embodiments comprise Fe-15 wt % Cr as a basic composition, and ferritic stainless steels were manufactured by changing the addition amount of Mo, W, and Nb, and [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)]. Hot annealing was partially performed with 20 mmt and 5 mmt, and coils having a thickness of 2.0 mm and samples having 20 mmt bar were manufactured by a hot annealing process and a cold annealing process. The samples manufactured as described above were the first Embodiments 1 to 7 and Comparative embodiments 1 to 4 in Table 1.

TABLE 1 ((Ti + Mo + ½Nb)/ C Si Mn P S Cr Mo W Ti Nb N Al Mo + W 0.83W (C + N)) invention 0.005 0.285 0.985 0.027 0.0014 14.9 0.51 4.54 0.105 0.43 0.005 0.07 5.05 4.28 32.00 steel 1 invention 0.0043 0.136 0.988 0.0281 0.0003 14.923 0.74 3.816 0.0921 0.442 0.0063 0.065 4.556 3.91 29.54 steel 2 invention 0.006 0.149 0.976 0.0238 0.0014 14.94 0.522 4.599 0.106 0.384 0.0077 0.072 5.121 4.34 21.75 steel 3 invention 0.005 0.282 0.923 0.027 0.0002 14.7 0.503 5.17 0.102 0.42 0.005 0.07 5.673 4.79 31.20 steel 4 invention 0.0066 0.189 0.976 0.0272 0.0014 14.8 0.704 5 0.0898 0.381 0.0076 0.062 5.704 4.85 19.74 steel 5 invention 0.005 0.21 1.01 0.027 0.0013 15.1 0.76 3.5 0 0.504 0.0064 0.081 4.26 3.67 22.11 steel 6 invention 0.0046 0.294 0.961 0.028 0.0014 14.95 0.7 3.6 0.11 0.41 0.0068 0.058 4.3 3.69 27.63 steel 7 comparative 0.006 0.298 1 0.028 0.0012 15 0.495 0 0.107 0.42 0.0049 0.05 0.495 0.50 29.08 steel 1 comparative 0.005 0.301 1 0.028 0.0013 15.23 1.51 0 0.108 0.43 0.0049 0.056 1.51 1.51 32.63 steel 2 comparative 0.005 0.298 0.986 0.027 0.00134 15 0.552 1.02 0.106 0.43 0.0048 0.061 1.572 1.40 32.76 steel 3 comparative 0.0076 0.439 0.884 0.0231 0.0006 18.24 1.94 0 0.125 0.463 0.0062 0.043 1.94 1.94 25.83 steel 4

2. Property Test of Ferritic Stainless Steel at Room Temperature and High Temperature.

As shown in Table 2, a room-temperature tensile strength test and thermal fatigue lifespan, oxidation resistance, and salt corrosion resistance were performed at high temperature in order to check high-temperature properties of the components, in Embodiments 1 to 7 and Comparative embodiments 1 to 4, which were manufactured, as shown in Table 1.

First, thermal fatigue samples were manufactured by machining the samples according to Table 1. Thermal fatigue lifespan was tested within the temperature range of 200-900° C. and a confinement factor of 0.3 by using the thermal fatigue samples manufactured as described above. Further, Embodiments 1 to 7 and Comparative embodiments 1 to 4 were heated at 1000° C. for 200 hours to test the oxidation resistance. Changes in weight were measured and the oxidation resistance at high temperature were checked, after cleaning and removing an oxidized scale produced by heating, with acid. Solution of 26% NaCl was made to test the salt corrosion resistance. Impregnating the samples according to Embodiments 1 to 7 and Comparative embodiments 1 to 4 with the solution of 26% NaCl for 5 minutes after keeping at 500° C. for 2 hours were performed ten times, and then reduction of weight was measured and the salt corrosion resistance at high temperature was tested.

The following Table 2 shows the test results of the embodiments and the comparative embodiments according to Table 1 of room-temperature tensile strength, r-bar value, thermal fatigue lifespan, high-temperature oxidation resistance, and high-temperature salt corrosion resistance.

TABLE 2 room- temperature thermal tensile fatigue high-temperature high-temperature salt strength lifespan oxidation resistance corrosion resistance % r-bar value (cycle) (mg/cm2) judgment (g/mm2) judgment invention 35.2 0.906 2250 8.077 ∘ 2.996 ∘ steel 1 invention 32.6 0.945 2260 10.1 ∘ 2.823 ∘ steel 2 invention 38.5 1.015 1860 8 ∘ 2.996 ∘ steel 3 invention 32.3 0.91 7.93 ∘ 2.6 ∘ steel 4 invention 31.5 0.932 2520 7.8 ∘ 2.61 ∘ steel 5 invention 31.1 1.01 10.56 ∘ 2.6 ∘ steel 6 invention 31.9 1.019 10.4 ∘ 2.7 ∘ steel 7 comparative 37.4 1.318 12.1 x 20.2 x steel 1 comparative 37.8 1.166 1320 4.603 x 18.288 x steel 2 generating scale comparative 38.4 1.175 1310 5.21 x 18.1 x steel 3 generating scale comparative 30 0.955 1542 11.118 x 14.378 x steel 4

Referring to Tables 1 and 2, it could be seen that the room-temperature tensile strength and r-bar values that are factors estimating ease of forming both satisfied predetermined desired values in Embodiments 1 to 7 and Comparative embodiments 1 to 4. On the other hand, it could be seen that although the thermal fatigue lifespan, high-temperature oxidation resistance, and high-temperature salt corrosion resistance, that are factors for testing properties at high temperature satisfied predetermined desired values for the exhaust manifold of a vehicle in Embodiments 1 to 7, the test items failed to be satisfied in Comparative embodiments 1 to 4.

All of Embodiments 1 to 7 according to the present disclosure comprise Mo of which the addition amount is 0.8 wt % or less. Further, all of Embodiments 1 to 7 comprise W of 3 wt % or more to 6 wt % or less. In this case, it could be seen that the room-temperature and the r-bar value were excellent, at 31% or more and 0.9 or more, respectively, which are items that makes it possible to test formability of ferritic stainless steel, in the embodiments of the present disclosure. Further, it could be seen that the thermal fatigue lifespan of Embodiments 1-3 and 5 according to the present disclosure were 2250, 2260, 1860, and 2520, respectively, which all satisfied 188 cycle or more.

In the test of high-temperature oxidation resistance in Embodiments 1 to 7, it could be seen that the maximum of weight changes was 10.56 mg/cm2 (Embodiment 6) and the minimum was 7.8 mg/cm2 (Embodiment 5). On the other hand, it could be seen that scale separation occurred in Comparative embodiments 2 and 3 of Comparative embodiments 1 to 4.

For the high-temperature salt corrosion resistance, it could be seen that the maximum was 2.996 g/mm2 and the decrease in weight was 3 g/mm2 in Embodiments 1 to 7, whereas the maximum was 20.2 g/mm2 and the minimum was 14.378 g/mm2 in Comparative embodiments 1 to 4 (Embodiment 4), that is, the decrease in weight was increased in comparison to Embodiments 1 to 7.

According to the test results, it could be seen that the room-temperature tensile strength and r-bar value, which are properties of formability at room temperature, show substantially equivalent values in the embodiments and the comparative embodiments, whereas the thermal fatigue property, high-temperature oxidation resistance, and high-temperature salt corrosion resistance of the comparative embodiments, which are high-temperature properties, were decreased, as compared with the embodiments. For the high-temperature oxidation resistance in Embodiments 1 to 7, the change in weight was 11 mg/cm2 or less and all of the samples satisfied the high-temperature oxidation resistance. On the other hand, for the comparative embodiments, the high-temperature oxidation resistance failed to satisfy predetermined desired conditions in Comparative embodiments and 4, and in addition, a scale is separated in Comparative embodiments 2 and 3. Further, in the high-temperature salt corrosion resistance repetitively tested, it could be seen that the decrease in weight was 3 g/mm2 or less in Embodiments 1 to 7, while the maximum of the decrease in weight was 20.2 g/mm2 in the comparative embodiments, such that all the four comparative embodiments failed to satisfy the high-temperature salt corrosion resistance.

It could be seen that all Embodiments 1 to 7 according to the present disclosure satisfied the properties at high temperature, in addition to formability at room temperature. Therefore, the ferritic stainless steel according to the present disclosure is available at high temperature and the formability can also satisfy predetermined conditions. Therefore, it could be seen that the embodiments can be used for the exhaust manifold of vehicles which requires formability.

The ferritic stainless steel according to the present disclosure comprises a smaller amount of Mo that is an expensive raw material and has properties suitable for an exhaust manifold, such that it is possible to reduce the manufacturing cost. On the other hand, in the comparative embodiments that fail to satisfy the content of Mo and W, it could be seen that the thermal fatigue lifespan, high-temperature oxidation resistance, and high-temperature salt corrosion resistance, which are properties at high temperature, were not satisfied.

Further, referring to Tables 1 and 2, it was shown that the samples manufactured by controlling the value of Mo wt %+0.83W wt % at 3.5% or more to 5% or less and the value of [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)] at 19.5 or more to 32 or less satisfy all the properties, because the high-temperature properties and high-temperature formability are both excellent.

FIG. 1 is a graph showing a test result of an influence of Mo and W on high-temperature strength.

Referring to FIG. 1, by weight %, C: 0.005 wt %, N: 0.0006 wt %, Cr: 15 wt %, Nb: 0.4 wt % and Ti: 0.1 wt % were contained in all the cases, and the influence of Mo and W on the high-temperature strength of ferritic stainless steel was tested while changing the addition amount of Mo and W.

First, the addition amounts of Mo-added steel and Mo+W-added steel were changed and the high-temperature tensile strength was tested at 900° C., in order to test influence of Mo and W on the high-temperature strength. X-axis shows the addition amount of Mo (Mo-added steel) or the addition amount of Mo+W (Mo+W-added steel) and Y-axis shows the corresponding high-temperature tensile strength in the graph, and the relational formula was acquired. The relationship with the addition amount of Mo or Mo+W that influences the high-temperature strength was shown by using a regression equation. As a result, a relational formula of high-temperature tensile strength at 900° C. (MPa)=22.4+4.7Mo+3.9W could be acquired, and accordingly, it could be seen that the contribution degree of W influencing the high-temperature strength was 83% (W/Mo=3.9/4.7=0.83).

FIG. 2A is an optical microscopic picture of a hot-annealed structure of Mo-added steel and FIG. 2B is an optical microscopic picture of a hot-annealed structure of Mo+W-added steel.

FIGS. 2A and 2B are optical microscopic pictures comparing sigma phases in the hot-annealed structures of the Mo-added steel and the Mo+W-added steel, respectively. FIG. 2A shows a hot-annealed structure of ferritic stainless steel containing Mo of 3 wt %, wherein the sigma phase existed up to 20% in the hot-annealed structure of the ferritic stainless steel. FIG. 2B shows ferritic stainless steel containing Mo of 0.5 wt % and W of 4.5 wt %, wherein the hot-annealed structure of the ferritic stainless steel includes a sigma phase of 5% or less. Referring to FIGS. 2A and 2B, it could be seen that it is possible to control the sigma phase of the hot-annealed structure at a lower percentage in the Mo+W-added steel than the Mo-added steel.

FIG. 3 is a graph testing thermal fatigue properties of ferritic stainless steel according to the addition amount of Mo and W.

Referring to FIG. 3, it could be seen that the thermal fatigue lifespan at 900° C. was excellent in the embodiments in comparison to the comparative embodiments. That is, the X-axis is the high-temperature tensile strength in MPa at 900° C. and Y-axis is the progress cycle of the thermal fatigue lifespan, it could be seen that the values for the embodiments are all positioned at the upper right side in the graph, whereas the values for the comparative embodiments are positioned at the lower left side in the graph. That is, it could be seen that the high-temperature tensile strength and thermal fatigue lifespan at 900° C. were excellent in the embodiments, while they were relatively low in the comparative embodiments.

FIG. 4 is a graph testing high-temperature oxidation resistance of ferritic stainless steel according to the addition amount of Mo and W.

Referring to FIG. 4, two items with a relatively small change in weight generated scale on the surface in the comparative embodiments, and it could be seen that a larger change in weight was shown in the high-temperature oxidation resistance result in other comparative embodiments, except for the above comparative embodiments, as compared with the embodiments. That is, it could be seen that the embodiments showed excellent high-temperature oxidation resistance, as compared with the comparative embodiments.

FIG. 5 is a graph testing high-temperature salt corrosion resistance of ferritic stainless steel according to the addition amount of Mo and W.

FIG. 5 is a graph showing the test result of high-temperature salt corrosion resistance according to the addition amount of Mo or Mo+W, and corresponding changes in weight. It could be seen that as the addition amount of Mo or Mo+W increases, the change in weight was about 11 g/cm2 or less at about 4 wt %, while when the addition amount of Mo or Mo+W is about 2 wt % or less, the change in weight was about 14 g/cm2 or more. That is, it could be seen that the addition amount of Mo+W can influence the high-temperature salt corrosion resistance of the ferritic stainless steel while the change in weight for the high-temperature salt corrosion resistance is better in the embodiments compared with the comparative embodiments.

(Manufacturing Method)

FIG. 6 is a flowchart schematically illustrating a fabrication method of a ferritic stainless steel according to one embodiment of the present disclosure.

Referring to FIG. 6, a fabrication method of a ferritic stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe and other inevitable impurities may comprise providing a slab (S1), heating the slab (S2), hot annealing (S3), cold annealing (S4), and cold rolling (S5).

Heating the slab (S2) may be performed at a slab heating temperature that is 1180° C. or more to 1240° C. or less, with respect to slab temperature. When the heating temperature is less than 1180° C. in heating the slab (S2), sticking, wherein the ferritic stainless steel sticks to a roll and the surface of the ferritic stainless steel is removed, may occur in the hot rolling. Further, when the heating temperature of the slab is above 1240° C., the grain size of the ferritic stainless steel may be increased, such that toughness and r-bar value may be reduced. Therefore, it is possible to make the grain of the ferritic stainless steel fine size by controlling the heating temperature of the slab at 1180° C. or more to 1240° C. or less, such that it is possible to ensure formability and machinability by improving the toughness and r-bar value.

In hot annealing (S3), the hot-annealing temperature may be 1020° C. or more to 1070° C. or less, with respect to strip temperature.

In hot annealing (S3), the hot-annealing temperature may be within the range where the ferritic stainless steel is recrystallized in annealing. The hot annealing may be at as low temperature as possible, within the temperature range where recrystallization occurs. The lower the hot-annealing temperature, the more the recrystallized grains of the ferritic stainless steel may be implemented in fine size after hot annealing, and accordingly, the r-bar value of the ferritic stainless steel that has finally undergone cold annealing may show excellent characteristic. When the hot-annealing temperature is less than 1020° C. in hot annealing (S3), recrystallization of the ferritic stainless steel may be insufficient, such that formability and elongation may be decreased. Further, when the hot annealing temperature is above 1070° C., toughness of the ferritic stainless steel may decrease after hot annealing, such that a plate may be broken in the manufacturing process, or the grain size of the cold-annealed ferritic stainless steel may increase, such that a defect of orange peel may occur in forming. Therefore, it is possible to improve the toughness and r-bar value of the ferritic stainless steel by performing hot annealing (S3) with hot-annealing temperature at 1020° C. or more to 1070° C. or less, with respect to strip temperature.

The grain size of the ferritic stainless steel may be ASTM No. 3 or more. As value in ASTM No. increases, the ferritic stainless steel has a more fine grain size. Therefore, the larger the grain size, the more the fine grain size is provided. When the grain size is less than 3 in ASTM No., the grain size of the ferritic stainless steel increases, such that the ferritic stainless steel may become easily brittle in hot annealing and a defect, such as plate break, may be generated.

In cold annealing (S4), the cold-annealing temperature may be 1030° C. or more to 1080° C. or less, with respect to strip temperature.

In cold annealing (S4), when the cold-annealing temperature is less than 1030° C., recrystallization may be insufficient in cold annealing, such that the elongation and formability of the ferritic stainless steel may be decreased. Further, when the cold-annealing temperature is above 1080° C., the grain size of the ferritic stainless steel may increase and a defect of orange peel may be generated in forming. Therefore, in order to improve the high-temperature strength by making an extract of the ferritic stainless steel fine, the cold-annealing temperature may be 1030° C. or more to 1080° C. or less with respect to the strip temperature.

The hot-annealing temperature of the hot annealing (S3) and the cold-annealing temperature of the cold annealing (S4), (cold-annealing temperature)/(hot-annealing temperature) may be 1.0 or more to 1.1 or less.

The manufacture of a ferritic stainless steel according to the present disclosure may comprise hot annealing (S3) and cold annealing (S4) and the temperature of the hot annealing (S3) and the temperature of the cold annealing (S4) may influence each other. Although the formability and r-bar value may be improved and the high-temperature strength may be improved when the cold-annealing temperature increases, when the (cold-annealing temperature)/(hot-annealing temperature) is less than 1.0 in the relationship of the cold-annealing temperature and the hot-annealing temperature, the r-bar value of the cold-annealed ferritic stainless steel sheet may decrease and the formability may be decreased. Further, when the cold-annealing temperature increases and the (cold-annealing temperature)/(hot-annealing temperature) is above 1.1, the grain size may increase and a defect of orange peel may be generated in forming the ferritic stainless steel. Therefore, (cold-annealing temperature)/(hot-annealing temperature) may be 1.0 or more to 1.1 or less to increase the r-bar value and the high-temperature tensile strength.

Cold rolling (S5) may be performed at room temperature.

DBTT(° C.) may be 90° C. or less in the ferritic stainless steel.

The value T which is defined as by “T=−184.6+3.2(Cr wt %)+27.5(Mo wt %)+4243.4(C wt %+N wt %)−295.6(Al wt %)+0.9[Nb wt %/(C wt %+N wt %)]” may be 90 or less in the ferritic stainless steel.

The DBTT(° C.) and T are factors that are more advantageous as they become lower, when DBTT(° C.) is above 90° C. or T is above 90, the ferritic stainless steel can be easily broken, such that a defect, such as plate break, may be generated.

In the ferritic stainless steel, when DBTT(° C.) is above 90° C. or T is above 90, plate break may occur at welded portions that may be formed by laser welding and seam resistance welding in hot annealing (S3) and cold annealing (S4) by deterioration of the toughness in the manufacturing process. Further, since non-pressed portions and plate break may occur by deterioration of toughness at room temperature in cold rolling (S5) in the ferritic stainless steel, DBTT(° C.) may be 90° C. or less and T may be 90 or less to prevent the above and ensure the toughness of the stainless steel.

In one aspect, the present disclosure provides a ferritic stainless steel having thermal resistance at temperature of 900° C. or more, high-temperature tensile strength of at least 40 MPa, and formability equivalent to or more of 444 steel. In another aspect, the present disclosure provides a fabrication method of a ferritic stainless steel sheet having thermal resistance at temperature of 900° C. or more, high-temperature tensile strength of at least 40 MPa, and formability equivalent to or more of 444 steel.

The high-temperature tensile strength of ferritic stainless steel can be improved due to solid solution hardening, by adding elements having relatively large atomic radius, such as Nb, Mo, W, Ta, and Hf. The one having the most excellent high-temperature tensile strength in the ferritic stainless steels used for thermal resistance is 444 steel, which contains Mo of 2 wt % or less. In the ferritic stainless steel where only Mo is added, the improvement effect in high-temperature strength is not so large even if the addition amount of Mo is increased up to 3 wt % or more. Further, when the addition amount of Mo is increased, a sigma phase is easily extracted in the manufacturing process of the ferritic stainless steel, such that a defect, such as plate break or orange peel, is increased by deterioration of toughness of the mother material and the welded portions, thereby reducing efficiency in manufacturing.

Therefore, in one aspect, the present disclosure provides an alloy design that can use fine laves phases and is based on ferritic stainless steel of which the addition amount of Mo is decreased and the addition amount of W is increased, in consideration of rapid extraction of laves phases as compared with Mo. In yet another aspect, the present disclosure provides a fabrication method of a ferritic stainless steel that prevents a defect in the manufacturing process which may be caused by deterioration of welded portions or the welded portion of a coil, and includes hot-annealing, cold-annealing, and cold rolling.

DBTT (° C.) was acquired by machining the samples (2.0 t) of Embodiments 1 to 7 and Comparative embodiments 1 to 4 according to Table 1 into V-notch impact samples and performing impact tests. It could be seen that the DBTT (° C.) of Embodiments 1 to 7 according to Table 1 was 90° C. or less.

3. Property Test of Ferritic Stainless Steel According to Temperature

A sample was manufactured by using the ferritic stainless steel according Embodiment 2 of Table 1 and changing the heating temperature in the step of heating a slab to 1230° C. and 1280° C.

FIG. 7 is a graph showing the grain size of ferritic stainless steel according to slab heating temperature and FIG. 8 is average r-bar values of ferritic stainless steel according to slab heating temperature.

FIG. 7 shows a graph showing the grain size of ferritic stainless steel that was manufactured at different heating temperatures in a step of heating a slab and then hot-annealed at 1050° C. It could be seen that the grain size of the hot-annealed ferritic stainless steel according to the slab heating temperature may be influenced by the heating temperature of the slab. In the hot-annealed ferritic stainless steel, the grain size is large when the slab heating temperature was at 1280° C., above 1240° C., and the grain size was small when the slab heating temperature was lower, at 1230° C.

FIG. 8 is a graph showing average r-bar values of ferritic stainless steel that was manufactured at different heating temperatures in a slab heating step, hot-annealed at 1050° C. and then cold annealed. It could be seen that as the heating temperature of the slab was decreased, the average r-bar value of the cold-annealed ferritic stainless steel was increased.

On the basis of the above description, it could be seen that when the heating temperature was decreased in the slab heating step, the grain size of the ferritic stainless steel was decreased and the average r-bar value was increased.

FIG. 9 is a graph showing average r-bar values according to hot-annealing temperature in hot annealing and FIG. 10 is a graph showing high-temperature tensile strength according to cold-annealing temperature in cold annealing.

Referring to FIG. 9, the average r-bar value of the ferritic stainless steel was measured while changing the hot-annealed temperature to 1040° C. and 1280° C. in the step of hot annealing. As the result of measuring, it could be seen that as the lower the temperature in the step of hot annealing, the larger the r-bar value.

FIG. 10 shows the result of measuring the high-temperature tensile strength of the ferritic stainless steel according to the cold-annealing temperature in the step of cold annealing. The high-temperature strength was measured at 900° C. and the cold-annealing temperature was changed to 1030° C. and 1060° C. Referring to the result, it could be seen that when the cold-annealing temperature was 1060° C., the high-temperature tensile strength was increased in the ferritic stainless steel.

FIG. 11 is a graph showing average r-bar values according to cold-annealing temperature/hot-annealing temperature and FIG. 12 is a graph showing high-temperature tensile strength according to cold-annealing temperature/hot-annealing temperature.

Referring to FIG. 11, it could be seen that as the cold-annealing temperature/hot-annealing temperature was increased, the average r-bar value of the ferritic stainless steel increased, which was measured by changing the cold-annealing temperature/hot-annealing temperature after cold annealing. That is, the average r-bar value increased, as the cold-annealing temperature to the hot-annealing temperature increased. On the other hand, it could be seen that when the cold-annealing temperature/hot-annealing temperature was above 1.1, a defect of orange peel was generated in the ferritic stainless steel.

FIG. 12 shows values of high-temperature tensile strength measured by changing the cold-annealing temperature/hot-annealing temperature. The high-temperature tensile strength was measured at 900° C. It could be seen that the high-temperature tensile strength was increased, as the cold-annealing temperature/hot-annealing temperature, which was an annealing temperature ratio in the step of hot annealing and the step of cold annealing, became higher in the ferritic stainless steel.

It will be understood to those skilled in the art that the present disclosure may be implemented in various ways without changing the spirit of the present disclosure. Accordingly, the disclosure described herein should not be limited based on the described embodiments. 

1. A ferritic stainless steel comprising: by weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe and other inevitable impurities.
 2. The ferritic stainless steel according to claim 1, wherein Mo is 0.8 wt % or less, by weight %.
 3. The ferritic stainless steel according to claim 2, wherein a hot-annealed structure of the ferritic stainless steel comprises a sigma phase of 5% or less.
 4. The ferritic stainless steel according to claim 1, wherein W is 3 wt % or more to 6 wt % or less, by weight %.
 5. The ferritic stainless steel according to claim 1, wherein Mo wt %+0.83W wt % is 3.5 wt % or more to 5 wt % or less, by weight %.
 6. The ferritic stainless steel according to claim 1, wherein [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)] is 19.5 or more to 32 or less.
 7. The ferritic stainless steel according to claim 1, wherein ductile-brittleness transition temperature (DBTT) is 90° C. or less.
 8. The ferritic stainless steel according to claim 1, wherein the ferritic stainless steel satisfies the following equation: −184.6+3.2(Cr wt %)+27.5(Mo wt %)+4243.4(C wt %+N wt %)−295.6(Al wt %)+0.9[Nb wt %/(C wt %+N wt %)]
 90. 9. A fabrication method of a ferritic stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.01 wt % or less; and the balance of Fe and other inevitable impurities, wherein the fabrication method of the ferritic stainless steel sheet comprises: providing a slab; heating the slab; hot annealing; cold annealing; and cold rolling.
 10. The fabrication method of a ferritic stainless steel according to claim 9, wherein Mo is 0.8 wt % or less, by weight %.
 11. The fabrication method of a ferritic stainless steel according to claim 9, wherein W is 3 wt % or more to 6 wt % or less, by weight %.
 12. The fabrication method of a ferritic stainless steel according to claim 9, wherein: Mo wt %+0.83W wt % is 3.5 wt % or more to 5 wt % or less, by weight %.
 13. The fabrication method of a ferritic stainless steel according to claim 9, wherein: [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)] is 19.5 or more to 32 or less.
 14. The fabrication method of a ferritic stainless steel according to claim 9, wherein: ductile-brittleness transition temperature (DBTT) is 90° C. or less.
 15. The fabrication method of a ferritic stainless steel according to claim 9, wherein the ferritic stainless steel satisfies the following equation: −184.6+3.2(Cr wt %)+27.5(Mo wt %)+4243.4(C wt %+N wt %)−295.6(Al wt %)+0.9[Nb wt %/(C wt %+N wt %)]<90.
 16. The fabrication method of a ferritic stainless steel according to claim 9, wherein grain size is ASTM No. 3 or more.
 17. The fabrication method of a ferritic stainless steel according to claim 9, wherein heating the slab has slab heating temperature that is 1180° C. or more to 1240° C. or less, with respect to slab temperature.
 18. The fabrication method of a ferritic stainless steel according to claim 9, wherein hot annealing has a hot-annealing temperature that is 1020° C. or more to 1070° C. or less, with respect to strip temperature.
 19. The fabrication method of a ferritic stainless steel according to claim 9, wherein cold annealing has a cold-annealing temperature that is 1030° C. or more to 1080° C. or less, with respect to strip temperature.
 20. The fabrication method of a ferritic stainless steel according to claim 9, wherein (cold-annealing temperature)/(hot-annealing temperature) is 1.0 or more to 1.1 or less, and wherein the cold-annealing temperature is the temperature in cold-annealing and the hot-annealing temperature is the temperature in hot annealing. 